377sbc

I,ve just put together a 377 sbc, but it's not performing as expected.
It's 4.155x 3.48, 10.8cr, Comp Cam's solid roller 246 & 254 @.050",kiwi pro 220cc heads(I think these are sold as pro topline or pro action now),Victor Jnr with 825 BG carb, 1 3/4'extractors.
I was expecting 500+hp but it would be flat out making 400. My old 383 junker with little flat tappet cam and double hump fuellies was running quicker and only revving to 6000.
The 377 spins over 7000 with no sign of breaking down or misfire, but as soon as it has a load on it just hasn't got any grunt.
I've been back through everything checking and can't find any problems. Feels like it's running at half throttle.(yeh, I've checked for full thottle)
I had a similar combo to this before and it went a helva lot better than this with a similar sized solid flat tappet cam and ported fuellie heads.
Anyone got any HP estimations or ideas of what else to check?  

I would estimate 500-520 H.P. what is the timing set at, how much fuel pressure do you have at the carb,what gears are you running as well as what stall speed is the converter you are using.  
Professonal hi-performance engine builder

horsepower sells engines and torque wins races

What valvesprings are you running? Do they match the cam? A mismatched set of springs can cause the problems you're having.  

sounds like u have a too large intake port,thus effecting the port velocity.port velocity effects torque.i just finished building a similar 380,except i used AFR's eliminator 210 heads,and the comp extreme roller(254 intake 260@.050)super victor intake and a demon 825,made 620hp @ 6800 and 500 ft lbs @ 5000.  

i agree...heads are too large....IMHO i would of picked an AFR195 eliminator ....

nosnerd  

Timing is 18 initial & 18 centrifical, all in by 3500rpm. I have tried a locked up distributor, but it ran the same times.
Carb is 825 BG demon. Fuel is 98 octane unleaded.
I've tried differnt fuel pressure settings for 5 to 10 psi. The pumps are OK and fuel pressure is still up where it's set at the finish line.
Valve springs are 220lb on the seat and 500lb on the nose.
The torque convertor flares to 3400rpm, which I know is a bit tight for this engine. It was built for 383 on nitrous.
I understand what you are saying about port velocities and heads being too big, but if this was the case wouldn't the engine be lazy in the bottom end and perform good at high revs?
This is the part that is a bit confusing, as it 60fts the same as my 383 with no gas. I was expecting it to be slower in the 60ft with this convertor. This indicates that it is still making reasonable torque at low revs. More than I would have expected.
Also in the top end of the rev range, it still feels lazy. Makes no difference to times whether I shift at 6000 or 7200rpm.
When I ran it at the first meeting, I thought the cam timing was out. I pulled the engine out and checked the cam timing with degree wheel but it was all OK.  

your cam is to blame also. a small cam with large heads will give you the result that you are experiencing. after you factor in your lash,your cam only has 240 degrees at .050,so you are not going to make any power upstairs,especially with those heads.you need a cam with approx 256-260 intake duration at .050 and exhaust around 260-264 at 050,lobe center at 106-108 degrees on a 102 intake centerline.hope this helps.  

you really need to consider the weight of your vechile!a383 puts out a lot of torque which would give you good 60 ft times.as to where a 377 would  allow you to run a wider rpm range.but would not really make a diffrence in your time. i know this dosent help with your problem but you need to realize its a factor.if you havent fixed your problem i suggest you speak to a professional if i have a problem that i cant figure out i take the advice of my local machine shop after all who would know better! call the chevy shop in chicago they have been open for 47 years and they could help beleive me they are very good at what they do # is 773-777-1818ask for roger he can help  

I read that different brand cam and lifters aren't interchangable.
My 377 has Comp Cam roller with Crane Roller lifters.
Could this cause the HP loss that I'm having??

I reckon I'm short about 150HP.  

alittle confused you only have 18 degr timming total, or is it set at 18 deg initial, with 18 deg more of advance totaling 36 degrees?

is the rest of your ignition system good?
maybe you have an upper rpm miss. i never really spun an engine that high, but i have noticed a dead cylinder, or two really smoothes out as rpm increases.

don't listen to them on port velocity, it helps, but it's not prudent. if you think i'm full of it take a good look at the new ls1's they all have 210cc intake ports, even there little 4.6ltr.
ok that might not all be true but there 350 ls1 ports are 210cc with a small cam, and thats a fact.

it's in your tune somewhere. either fuel, or spark.  

18deg initial plus 18 deg centrifical. 36 total.

Yeh, I figured if the ports were too big it would be lazy at low revs nad get going at high revs when the airspeed was up if they were too oversized. But this isn't the case, It 60 fts the same as my old 383 junker that had small flat tappet cam and fuelie heads, And the old 383 runs about a tenth quicker in the 1/8th mile.  

A far as the lifters I never heard that one before the problem may be that the destroker is not making the torque that the 383 stroker was making have you tried turning it a little higher.  
Professonal hi-performance engine builder

horsepower sells engines and torque wins races

I agree the heads are too big.I have a 377 also and called AFR and they said to use the 195 runners of theirs.  

I understand about what people are saying that the heads are too big. But what I don't understand is the fact that they don't seem to exhibit to caracteristics of a head too large.
If they were too large, I would expect the engine to be lazy in the bottom end and go hard when the revs are high enough to get good airspeed.
Even revving it to 7500, it doesn't seem to come alive at any point and also the bottom end seems better than I would have expected with this combo. It 60fts similar to the old 383.
It runs flat and lazy through the whole rev range.
At a guess I would reckon it's about 100+hp on what I reckoned this combo should make.
On my engine program it comes out at 570HP.
I ran the Comp Cams "camquest" program. The top cam that it recommended was the next smaller one that I am using, and the one I have was their 2nd choice. Their program reckoned 580HP.
"Big Cam 406"wrote in an earlier post that he had built a similar combo with next size larger cam and 210cc heads that mado 620HP.
So with all this in mind, I reckon that 570hp should be somewhere in the ballpark. Even if it was a bit less, thats OK as I don't take computor simulations as gospel, but they should be somewhere in the ballpark.
I reckon this engine would be flat out making 400HP.
So if I am a little bit off in the choice of heads and cam, do you think it would be down by this much.  

I agree, heads are too big. Lets not forget ls heads are also a different configuration. Why have intake runners gotten so big? Oh yea, poor rod ratio. 150 or less on bigger sbc strokers. A 377 has a great rod ratio. Depending on the rod length your in the 1.7 plus range. With that you have velocity built in. Thats why a 327 ran so good. Small port heads with a 1.75 rod ratio. Your 3.77 with a 6 inch rod carries a 1.72 rod ratio. a 383 w/ a 5.7 rod has a 1.52 rod ratio. This makes for poor port velocity and a larger runner is used to make it up with volume.
Ask Grumpy Jenkins he was the master of the rod ratio game.